What is Aperture Science

Aperture science is a fictional corporation that exists in the Half-life universe. It is a competitor to Black Mesa. In the game portal you are trapped in an abandoned Aperture Science Enrichment Center, held hostage and forced to perform potentially lethal tests by the insane A.I. GLaDOS. GLaDOS took over the facility by killing most of the employees with deadly neurotoxin.
More on the History: http://half-life.wikia.com/wiki/Aperture_Science